1. Field of the Invention
The present invention relates to a beam irradiation apparatus for irradiating a target region with a laser beam and, particularly, to a beam irradiation apparatus mounted on a so-called laser radar, for detecting the presence or absence of an obstacle in a target region and a distance to an obstacle based on reflection light of a laser beam emitted to a target region.
2. Description of the Related Art
In recent years, a laser radar is mounted on a family car or the like in order to enhance safety during driving. The laser radar emits a laser beam to the front in the driving direction and detects the presence or absence of an obstacle in a target region and distance to an obstacle. Generally, the laser radar scans a target region with a laser beam and, based on the presence or absence of reflection light in each of scanned positions, detects the presence or absence of an obstacle in each of the scanned positions. Further, based on required time from a laser beam emission timing in each scan position to a reflection light reception timing, the distance to the obstacle from the laser radar in the scan position is detected.
To enhance detection precision of a laser radar, a target region has to be properly scanned with a laser beam, and each scan position of a laser beam has to be properly detected. As a laser beam scanning mechanism, a scan mechanism using a polygon mirror and a lens-driving-type scan mechanism for two-dimensionally driving a lens for scan are known.
On the other hand, as a method different from the scan mechanisms, a mirror-turning-type scan mechanism can be proposed. In the scan mechanism, a mirror is supported so as to be driven about two axes. The mirror is turned about each of the drive shafts as an axis by an electromagnetic drive force between a coil and a magnet. A laser beam is obliquely incident on the mirror. By two-dimensionally driving the mirror about each of the drive shafts as an axis, a target region is scanned in the horizontal and vertical directions with reflection light of the laser beam by the mirror.
In the scan mechanism, scan positions of the laser beam in the target region correspond to turn positions of the mirror in a one-to-one corresponding matter. Therefore, the laser beam scan position can be detected by detecting the turn position of the mirror. The turn position of the mirror can be detected by, for example, detecting the turn position of another member which turns in association with the mirror.
FIGS. 8A and 8B show a configuration example in the case of detecting the turn position of another member. FIG. 8A shows a configuration example of the case of using a translucent member having a parallel plate shape as another member, and FIG. 8B shows a configuration example of the case of using a mirror member as another member.
FIG. 8A shows a semiconductor laser 601, a translucent member 602, and a position sensing device 603 (PSD). A laser beam emitted from the semiconductor laser 601 is refracted by the translucent member 602 disposed slightly tilted with respect to the axis of the laser beam, and the refracted beam is received by the PSD 603. When the translucent member 602 rotates as shown by arrows, the path of the laser beam changes as shown by a dotted line in the diagram, and the reception position of the laser beam on the PSD 603 changes. Therefore, according to the laser beam reception position detected by the PSD 603, the turn position of the translucent member 602 can be detected.
FIG. 8B shows a semiconductor laser 611, a mirror member 612, and a position sensing device 613 (PSD). A laser beam emitted from the semiconductor laser 611 is reflected by the mirror member 612 disposed slightly tilted with respect to the axis of the laser beam, and the reflected beam is received by the PSD 613. When the mirror member 612 rotates as shown by arrows, the path of the laser beam changes as shown by a dotted line in the diagram, and the reception position of the laser beam on the PSD 613 changes. Therefore, according to the laser beam reception position detected by the PSD 613, the rotation position of the mirror member 612 can be detected.
When the mirror member 612 rotates only by an angle α as shown in FIG. 8B, the rotation angle of the laser beam reflected by the mirror member 612 is 2α. Consequently, the light reception surface of the PSD 603 has to be enlarged. On the other hand, when the translucent member 602 is used as shown in FIG. 8A, even when the translucent member 602 rotates, the shift width of the laser beam passed through the translucent member 602 is not large. Therefore, as compared with the case of FIG. 8B, the light reception surface of the PSD 603 can be made much smaller, and the cost of the PSD can be suppressed.
In the configuration of FIG. 8A, the semiconductor laser 601 is normally controlled so that its emission power becomes constant. Generally, the power control is performed based on an output from a PD (Photo Detector) for a monitor in a laser package. That is, the emission power of the semiconductor laser 601 is controlled so that an output from the PD for a monitor has a predetermined magnitude.
In the case of making the translucent member 602 rotate as described above, in association with the rotation, the amount of light reflected by the laser beam incident surface and the outgoing surface of the translucent member 602 changes. Consequently, when the outgoing power of the semiconductor laser 601 is constant, the amount of the laser beam received by the PSD 603 changes in association with the rotation of the translucent member 602. With the change, an error occurs in a position detection signal output from the PSD 603. The error exerts an influence on the detection precision of the scan position of the laser beam in the target region.